Tropomyosin can be induced to bind to F-actin by myosin heads in the form of heavy meromyosin, myosin subfragment-1, and heavy meromyosin which has been modified by N-ethylmaleimide. Heavy meromyosin induces binding between tropomyosin and F-actin under conditions of low ionic strength (12mM KC1). This behavior is in contrast to that of troponin-I which induces binding between tropomyosin and F-actin only under conditions of higher ionic strength.